The present invention relates to a projection lens system and particularly to a projection lens system with a wide field angle which provides a bright image having an excellent focus performance even in the marginal area, uses an inexpensive glass material, and has a short projection distance and a projection image display apparatus using the system which is excellent in cost performance.
Recently, a television set as an image display apparatus for home use is proceeding to a larger screen size as the wide aspect ratio increases. As an image display apparatus for home use, there are two types available: a direct view type using a cathode ray tube and a so-called projection type for enlarging and projecting an image from a miniature projection tube, whose screen size is about 7 inches of (diagonal) via a projection lens system. Due to restrictions concerning compactness and weight of a TV set, for a screen size of more than about 37 inch of diagonal, a projection image display apparatus is mainly used.
At first, this projection image display apparatus was inferior to the direct view type in screen brightness and focus performance. However, recently, the performance of each of the components such as the projection lens system, screen, and projection tube is improved, so that both the screen brightness and focus performance are approaching those of the direct view type. In the performance improvement process of the projection image display apparatus, various arts have been developed in the projection lens system which is a key device. First, to obtain screen brightness equivalent to that of the direct view type or higher, as disclosed in U.S. Pat. No. 4,682,862, reduction of the F-number has been tried by using many plastic aspherical lens elements.
Second a projection lens system has been developed for realizing improvement of screen brightness and improvement of focus performance at the same time . With respect to this projection lens system, as disclosed in Japanese Patent Application Laid-Open No. 3-137610, there is an example using a plastic aspherical lens and a doublet glass lens. As a result, in the current projection TV set, a projection lens system having an F-number of about f/1.1 is used and both brightness and focus performance are improved on the whole screen.
At the third step, a projection lens system with a wide field angle by which a compact set dimension can be realized on account of the short projection distance has been developed mainly. A reference describing an actual art for realizing a projection lens system with a wide field angle without reducing the brightness and focus performance in the marginal area and an actual projection lens system is disclosed in Japanese Patent Application Laid-Open No. 4- 5608. Hereinafter, the art disclosed in this patent is referred to as a first prior art.
In this first prior art, by combining plastic aspherical lens elements and glass lens elements effectively in a projection lens system of six lens groups, the aforementioned problem is solved. Furthermore, the projection lens system is structured so that almost all the positive refractive power of the projection lens system is shared by the glass lenses and the plastic aspherical lens elements have little refractive power, so that the peculiar drift of the focus performance due to a temperature change is reduced even if the plastic aspherical lens elements are used is reduced.
In this first prior art, the profile of the fluorescent face of projection tube has a curvature so that it is convex on the electron gun side. As a result, the projection lens system is structured so that the normal of the fluorescent face in the marginal area is in the direction of the entrance pupil of the projection lens system and can fetch more light fluxes in comparison with the case using a flat fluorescent face. Therefore, even if the field angle is widened, a relative illuminance of a level which is almost no problem practically can be obtained in the marginal area.
The curvature of field is corrected by the lens element of the sixth lens group (hereinafter referred to as sixth lens). However, if the fluorescent face of projection tube has a curvature so that it is convex on the electron gun side, the generation amount of curvature of field is reduced and the focus performance in the marginal area is improved.
Furthermore, a projection lens system with a wide field angle which realizes a more excellent focus performance without reducing the brightness in the marginal area and an actual art for realizing it are disclosed in U.S. Pat. No. 5,272,540. Hereinafter, the art is referred to as a second prior art.
In the second prior art, a projection lens system having a constitution of five groups by six elements is disclosed and the profile of the fluorescent face of projection tube which is an object is an aspherical profile which is convex on the electron gun side. The curvature of the profile in the marginal area is smaller than that in the neighborhood of the optical axis. By doing this, highly precise correction of the curvature of field and astigmatism are compatible with each other and the satisfactory focus performance and the light amount which is sufficient practice are reserved in the marginal area of screen.
In this projection lens system, the lens element of the third lens group (hereinafter referred to as third lens) sharing almost all refractive power of the overall lens system, has a concave lens of large dispersion glass and a convex lens of small dispersion glass stuck together; the chromatic aberration is corrected, and the large aperture (the F-number is 0.96) and the high focus performance are compatible with each other. Furthermore, the combination of the lens element of the first lens group (hereinafter referred to as first lens) and the lens element of the second lens group (hereinafter referred to as second lens) offsets the lowering of the focus performance generated by deformation and expansion of each lens element due to temperature change and humidity change which is an intrinsic problem when plastic lens elements are used.
On the other hand, in a conventional projection lens system, as a lens barrel for assembling each lens element with high precision, a lens barrel having the constitution disclosed in, for example, Japanese Utility Model Application Laid- Open No. 2-51478 is often used. The lens barrel of the prior art has an outer barrel and an inner barrel which is installed inside the outer barrel and can slide in the direction of optical axis of the lens without axial shift. The inner barrel is constructed so that it can be divided into two parts longitudinally in the direction of diameter of the lens along the optical axis of the lens and it has slits for holding a plurality of lens elements at predetermined intervals with high precision on its inner surface.
In the aforementioned projection lens system having a constitution of six lens groups of the first prior art, there are several problems to be solved.
The first problem is caused by the lens constitution. In the aforementioned projection lens system, the third lens having negative refractive power is arranged on the screen side of the lens element of the fourth lens group (hereinafter referred to as fourth lens) sharing almost all the positive refractive power of the overall lens system. The spherical aberration and coma aberration are corrected by the third lens.
Therefore, the location of the entrance pupil of the overall lens system moves to the screen side from the center of the fourth lens. As a result, if an attempt is made to realize a wider field angle (reduction of the projection distance) in the aforementioned lens constitution, correction of the distortion and astigmatism becomes difficult.
Next, the second problem is a point that if an attempt is made to reduce the F-number or (increase the aperture ratio) of the projection lens system having this lens constitution and obtain a sufficient marginal light amount ratio, the apertures of the first, second, and third lenses become larger and the production cost increases.
The share of correction of each lens group in aberration correction of the aforementioned projection lens system is shown below.
The first lens is a spherical lens element of a meniscus profile having positive refractive power and corrects spherical aberration and coma aberration.
The second lens is a plastic aspherical lens element of a meniscus profile having weak positive refractive power and corrects spherical aberration and coma aberration.
The third lens is a spherical lens element having a weak divergent action and corrects spherical aberration and coma aberration.
The fourth lens is a convex-convex glass spherical lens element having a strong convergent action.
Furthermore, the lens element of the fifth lens group (hereinafter referred to as fifth lens) is a plastic aspherical lens element of a meniscus profile having weak positive refractive power and corrects astigmatism, distortion, and coma aberration.
The sixth lens has a constitution that it has a concave surface facing the screen side, has negative refractive power accompanied by a liquid coolant (A), and corrects curvature of field.
Among them, the second lens and fifth lens are a plastic aspherical lens element and have a meniscus profile having weak positive refractive power respectively. This projection lens system of prior art has a constitution that each plastic lens element has little refractive power and the peculiar shift of the focus performance due to a temperature change when the plastic aspherical lens element is used is reduced.
There is a third problem imposed that as mentioned above, in the projection lens system using the first prior art, the applicable lens profile of the plastic aspherical lens is limited to a specific profile and that the aberration correction cannot be attained sufficiently.
A fourth problem is also imposed that since four glass lens elements are included, the cost is increased.
Furthermore, the aspherical surface of the fifth lens is small, and the sixth lens is a glass lens element, whose screen side surface is spherical; so that correction of astigmatism and correction of curvature of field are not compatible with each other.
Therefore, a fifth problem arises that correction of astigmatism in the marginal area is difficult.
In the first prior art, it is a subject (of the design) to solve these problems.
A problem of the projection lens system having a constitution of five groups by six elements to be solved in the second prior art is reduction in cost.
The two factors for an increase in the cost of the projection lens system are described below.
The first factor for an increase in cost is the profile of fluorescent face of the projection tube. The main profile of fluorescent face of the projection tube is a spherical fluorescent face at present. When this projection lens system is applied, it is necessary to make the profile of fluorescent face aspherical and the projection tube is to be produced under a special specification, so that it is a factor for an increase in the cost of the set.
The second factor for an increase in cost is that it is essential to use a doublet lens comprising a large dispersion concave lens with a large diameter and a small dispersion convex lens with a large diameter which are stuck together for the third lens so as to realize a large aperture ratio (the F-number is 0.96) in this projection lens system and correct chromatic aberration satisfactorily.
Generally, the price of optical glass increases as the refractive index increases and as the dispersion decreases. In the second prior art, the optical glass used as a third lens of the projection lens system described in Embodiment 1 includes large dispersion glass of SF11 and small dispersion glass of SK16. The prices of these optical glass materials are more than 2 times as expensive as the price of SK5 which is typical of optical glass used in the projection lens system such that the price is 2.3 for SF11 and is 2.1 for SK16 (those glass names are abbreviations of Schott, Ltd. and often used in this field).
On the other hand, a problem when the aforementioned conventional lens barrel is used in the projection lens system is that the air temperature in the sealed space inside the lens barrel and the temperature of the lens elements rise, and the heated lens elements expands and deforms, and the focus performance of the projection lens system is lowered extremely.
As a result, it is a subject (of the design) to suppress rising of the air temperature in the sealed space inside the lens barrel and the temperature of the lens elements and to prevent the lens elements from expansion and deformation even if the heat generated from an image generating source is high.
An object of the present invention is to solve the problems of the projection lens system of the prior art mentioned above and to provide a projection lens system of a wide field angle which uses inexpensive optical glass, has an excellent focus performance even in the marginal area even if the heat generated from an image generating source is high, obtains a bright image, and has a short projection distance and a projection image display apparatus using the system which is excellent in cost performance.
To accomplish the above object, the projection lens system of the present invention uses technical means as described below.
Firstly, to solve the first and second problems of the first prior art, almost all the positive refractive power of the overall lens system is shared by the glass lens elements (hereinafter described as glass power lens). In this case, a lens having negative refractive power is not arranged on the screen side of the lens group including the glass power lens but a plastic aspherical lens element having weak positive refractive power around the optical axis is arranged there. As a result, the entrance pupil does not move to the screen side from the glass power lens, so that the first problem can be solved and a projection lens system with a wide field angle can be realized.
Furthermore, a light flux passing through the lens group including the glass power lens diverges and enters the lens groups positioned on the screen side, so that the aperture of each of lens groups can be made as small as possible and the second problem can be solved.
In the projection lens system of the present invention, to minimize the lowering of the focus performance due to temperature and humidity changes, the refractive power of the plastic aspherical lens element around the optical axis is reduced to 30% of that of the glass power lens or less.
The aberration depending on the aperture is corrected according to the profile of lens surface including aspherical system in the area (the marginal area of the lens element) apart from the optical axis. The system is structured so that the drift of the local refractive power obtained according to the profile of lens surface including aspherical system in the marginal area of the lens due to temperature and humidity changes is offset by combining a plurality of plastic aspherical lens elements. By doing this, the profile of lens element can be decided without affecting aberration correction restrictively and the third problem can be solved.
Many plastic aspherical lens elements having a lens surface including strong aspherical system can be used by the aforementioned technical means, so that the number of glass lens elements can be reduced and the fourth problem can be solved.
To solve the fifth problem, a lens element having negative refractive power with the concave surface facing the screen side is arranged in the location closest to the projection tube which is an image light source, and the lens surface of the lens element on the screen side is formed as an aspherical shape, and hence the astigmatism in the marginal area of the image is reduced. Furthermore, with respect to the lens element arranged on the screen side of this lens element, the profile of lens surface on the projection tube side is formed in a convex shape around the optical axis and a concave shape on the projection tube side in the marginal area and hence the astigmatism in the marginal area of the image can be reduced with higher precision.
To realize a reduction in cost which is a problem of the projection lens system having the constitution in the second prior art, the two following means are used.
The first means is to form the fluorescent face of a projection tube to be applied to the projection lens system as a spherical fluorescent face. When the fluorescent face of the projection lens system heaving a constitution of five group by six elements described in the first embodiment of the second prior art is changed to a spherical surface as it is, the length of optical path from an object point in the marginal area on the fluorescent face to the exit surface of the fifth lens is different between a beam of light passing through the saggital plane and a beam of light passing through the meridional plane, so that a great difference is generated in the focus performance between the saggital direction and the meridional direction because astigmatism conspicuously increases in the marginal area. This trend is specially significant in the marginal area between 90% of the distance (relative image height from center to corner) from the center of the screen to each corner and each corner.
Therefore, according to the present invention, the lens surface of the sixth lens having negative refractive power on the screen side is formed in a profile such that in the lens area traversed by the light flux from an object in the marginal area on the fluorescent face the lens action (divergent action) becomes weaker in comparison with that around the optical axis of the therefore, the difference between the length of optical path on the saggital plane and that on the meridional plane is reduced. Furthermore, when in the lens element arranged on the screen side of the above-mentioned lens element having negative refractive power, the profile of lens surface on the projection tube side is formed in a convex shape on the projection tube side around the optical axis and in a concave shape on the projection tube side in the marginal area, the difference of length of optical path can be made smaller and the astigmatism in the marginal area can be reduced remarkably.
The second means is to change the third lens to inexpensive optical glass.
For that purpose, correction of chromatic aberration is realized by a large dispersion plastic concave lens element and an inexpensive small dispersion glass convex lens.
It is also effective to install a filter for cutting the spurious component other than the dominant wavelength component among the light emission spectrum of a phosphorescent substance in at least one lens element of the lenses constituting the projection lens system and reduce the generated chromatic aberration itself.
Furthermore, to realize a large aperture, the aforementioned large dispersion plastic concave lens element is formed in a profile of strong aspherical shape and the aberration is corrected with higher precision. Furthermore, the lens profile is formed in a concave meniscus profile in which the concave surface faces the screen side around the optical axis and particularly in a profile that with respect to the lens surface on the projection tube side, the inclination of the lens surface in the marginal area of lens apart from the optical axis is increased. As a result, the entrance height of light flux into the third lens (glass) can be decreased and the diameter of the third lens (glass) can be made smaller when the same F-number is to be obtained, so that the cost can be reduced.
On the other hand, the projection lens system of the present invention is structured so that at least one communicating opening or communicating window extending outside of the projection lens system from the spaces between the lens elements is installed.
Furthermore, at least one space among the spaces between the lens elements is structured so that the communicating opening or communicating window is arranged individually in each of at least two leveling locations practically on the basis of the horizontal plane in the operation status of the projection lens system or continuously over those locations. In this case, the communicating opening or communicating window in the low location functions as an inlet of air and the communicating opening or communicating window in the high location functions as an outlet of air.
To install the communicating opening or communicating window, one of the methods (1) to (4) shown below is used or these methods are used together.
(1) Around the connection point of a lens element holding member for holding at least one lens element and covering the spaces among the lens elements and a connection member for connecting the lens element holding member to the image generating source, a communicating opening or communicating window is arranged as a space surrounded by at least the lens element holding member and the connection member. In this case, it is possible that the volume of this space is restricted by the size of protrusion provided in the lens element holding member or the size of protrusion provided in the connection member.
(2) A communicating opening or communicating window is arranged in the lens element holding member itself.
(3) A lens element holding member comprising a first holding member for holding at least one lens element and a second holding member for fitting and holding the first holding member is structured and a communicating opening or communicating window is arranged between the first holding member and the second holding member. In this case, at lease one groove provided in a concave shape on the inner side of the second holding member may be functioned as a communicating opening or communicating window.
(4) A communicating opening or communicating window is arranged around the periphery of the lens element.
When a communicating opening or communicating window is arranged by one of the aforementioned methods, it is desirable to set the space between the lens element arranged closest to the image generating source among a plurality of lens elements and the lens element second closest to the image generating source as a corresponding space.
The aforementioned communicating opening or communicating window is arranged so as to replace heated air in the spaces among the lens elements with air outside the projection lens system. In this case, new problems may arise that a foreign material such as dust enters from the communicating opening or communicating window and adheres to the lens elements, or an external light enters the projection lens system and the image contrast performance of the projection lens system is lowered, or when the projection lens system is used in a projection type image display apparatus, the image contrast performance of the projection type image display apparatus is remarkably lowered due to light leakage from the inside of the projection lens system.
To eliminate the problems, in the aforementioned projection lens system, in the opening portion of the communicating opening or communicating window toward the outside of the projection lens system, a dust-proof member, for example, a flange-shaped member is arranged in the way to protect the air permeability. Or, the communicating opening or communicating window itself is formed in a bent, or curved, or twisted shape.
In the projection lens system of the present invention, the aforementioned communicating opening or communicating window functions as an air inlet through which air at a low temperature (open air) is introduced and an air outlet through which heated air is ejected in the space among the lens elements in which the communicating opening or communicating window is provided. By doing this, the efficiency of heat radiation from the lens elements is increased by convection of air and the lens elements are suppressed in rising of temperature and hence the expansion and deformation due to rising of temperature are suppressed and as a result, the lens performance, particularly the focus performance are prevented from lowering.
When a projection type cathode ray tube is used as an image generating source, the projection type cathode ray tube becomes a heat generating source. Therefore, when the aforementioned communicating opening or communicating window is provided in the space between the lens element closest to the projection type cathode ray tube and the lens element second closest to it, the effect of the aforementioned action is remarkable. Air has a property that when it is heated, the specific gravity thereof decreases and it flows upward. Therefore, when the height of location of the communicating opening or communicating window which is used as an air outlet is set higher than the height of location of the communicating opening or communicating window which is used as an air inlet on the basis of a certain horizontal plane, a practically sufficient effect can be obtained in the aforementioned action.
On the other hand, when a dust-proof member is arranged in the opening portion of the communicating opening or communicating window toward the outside of the projection lens system when the communicating opening or communicating window itself is formed in a bent, or curved, or twisted shape, entry of a foreign material or light into the projection lens system and light leakage from the inside of the projection lens system can be prevented. As a result, the contrast performance of the projection lens system itself will not be lowered and neither will be readuced the image contrast of the projection type image display apparatus using the projection lens system.